This invention is directed to electro-optical devices whose electromagnetic radiation transmission characteristics can be selectively altered by the influence of a suitably controlled electrical field. More particularly this device relates to a system comprising electrochromic materials and conductive electrolyte which exhibit good chemical and electrochemical stability, reversibility and reproducibility. Still more particularly this invention is concerned with an improved counter electrode structure.
In prior U.S. Applications, Ser. No. 530,086, filed Feb. 25, 1966 abandoned and refiled as Ser. No. 616,791, Feb. 17, 1967; abandoned and refiled as Ser. No. 110,068, Jan. 27, 1971; abandoned and refiled as Ser. No. 348,878, Mar. 10, 1973, now U.S. Pat. No. 3,829,196, and Ser. No. 534,188, filed Mar. 14, 1966, now abandoned and refiled as Ser. No. 120,365, Mar. 2, 1971, abandoned and refiled as Ser. No. 315,966, Dec. 18, 1972, abandoned and refiled as Ser. No. 509,578, Sept. 26, 1974, now pending, and U.S. Pat. No. 3,521,941, there are described electro-optical devices exhibiting a phenomenon known as persistent electrochromism.
The term persistent electrochromism denotes the property of a material whereby its electromagnetic radiation absorption characteristic is altered, in most instances even at ambient temperature, under the influence of an electric field. Such materials, for example, may exhibit little or no absorption of visible wave lengths in the absence of an electric field and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, turning blue in color. Similar effects can be observed in other portions of the electromagnetic radiation spectrum, invisible as well as visible.
As described in the foregoing earlier applications, if a layer of a persistent electrochromic material is disposed between a pair of electrodes, across which a potential is applied, the radiation transmitting characteristic of the material will change. If the electrodes and the electrochromic layer as formed on the surface of a transparent substrate, such as glass, the light transmitting characteristics of the combination can be varied by controlling the electric field produced across the electrochromic material. On the substrate which originally is clear, i.e., presenting substantially no diminution of the light transmitting ability of the substrate, application of a voltage between the electrodes to establish an electric field of the proper polarity changes the light absorption characteristic of the electrochromic material, turning it darker, for example, thus decreasing the light transmitting ability of the entire assembly.
In copending, commonly assigned application, Ser. No. 41,154, filed May 25, 1970, abandoned and refiled Apr. 13, 1972, now pending is described the use of a large number of suitable metallic counter-electrodes. It has now been found that a very effective counter electrode structure is formed by the use of layers of plasticized persistent electrochromic material pressed onto each side of a metal screen.
It is therefore an object of this invention to provide a more effective counter electrode for an electrochromic cell.
This and other objects of the invention will become apparent as the description thereof proceeds.
As used herein, a "persistent electrochromic material" is defined as a material responsive to the application of an electric field of a given polarity to change from a first persistent state in which it is essentially non-absorptive of electromagnetic radiation in a given wave length region, to a second persistent state in which it is absorptive of electromagnetic radiation in the given wave length region, and once in said second state, is responsive to the application of an electric field of the opposite polarity to return to its first state. Certain of such materials can also be responsive to a short circuiting condition, in the absence of an electric field, so as to return to the initial state.
By "persistent" is meant the ability of the material to remain in the absorptive state to which it is changed, after removal of the electric field, as distinguished from a substantially instantaneous reversion to the initial state, as in the case of the Franz-Keldysh effect.